The present disclosure relates to bearing lubricating oils and bearings, and more particularly, to monoester lubricating oils used for fluid bearings, etc., and bearings using the same.
With the recent trend toward reducing the power consumption of audio devices, personal computers, etc. as well as downsizing, a hard disk drive etc. which are included in these devices are required to stop the motor in the standby state and rotate the motor only during access operation so as to reduce the power consumption. When the motor is frequently stopped and started, it is important to reduce the torque of the motor during start to reduce the power consumption. In order to achieve this, there is a need for a bearing lubricating oil (bearing oil) having low viscosity. Also, in order to allow the use of the hard disk drive etc. in cold climates, there is a need for a bearing oil with excellent low-temperature fluidity which does not solidify at as low as −40° C. On the other hand, the hard disk drive is required to operate at a temperature of about 50-60° C. In this use environment, the temperature of the bearing oil is likely to increase up to about 90° C. The temperature increase may induce a significant reduction in the viscosity of the bearing oil, leading to a reduction in bearing stiffness, so that the bearing does not work. Therefore, bearing oils for motors of hard disk drives etc. need not only to have low viscosity at low temperature, but also to keep the viscosity higher than a predetermined level at high temperature. Such bearing oils are required to have a high viscosity index, i.e., the viscosity decreases less with an increase in temperature.
Conventional bearing oils contain the following components: a diester, such as dioctyl sebacate; a polyolester composed of a polyol, such as trimethylolpropane etc., and a straight chain fatty acid containing from 4 to 8 carbon atoms; etc. Conventional bearing oils have, however, failed to satisfy characteristics which are recently required by hard disk drive motors etc. Therefore, a bearing oil containing a mixture of a plurality of diesters etc., a bearing oil containing a special diester etc., and the like are being studied (see, for example, Japanese Patent Publication Nos. 2004-91524 and 2005-154726).